INSTRUMENT DESIGN

Astronomical instruments are generally designed by a team of people
with expertise in optics, optical design, mechanical engineering,
electronics, software, control theory and other disciplines. The
design process normally involves a lengthy period before a preliminary
design is reviewed by the entire group. During this time, scientists
simulate what the instrument will do, how well it will perform and
what the requirements on the actual design will be. Discussions with
people at the observatory also provide constraints on the design:
things as simple as whether the instrument will fit through the
observatory's door, to the exact specification of the beam that will
be fed from the telescope to the instrument.

The team generates a preliminary design, which is then
reviewed,
usually over a single day's meeting of the entire group, plus some
additional scientists and engineers who are asked to be present in a
consultory role. The preliminary design review (a.k.a. PDR) usually
results in significant changes in the design. Problems are generally
found during the PDR. These issues are rectified during a secondary
design process, which can take months or even years in some instances.
Ultimately a second design review is held, called the critical design
review (CDR). At this point essentially every part of the instrument
has been identified and vendors that can either build or supply those
parts have been identified. If the critical design review reveals
significant flaws in the instrument construction plan or design,
another period of design revision ensues and another CDR will be
scheduled. Upon passing CDR, the instrument construction starts.

The Lyot Project coronagraph, which went through successful
CDR in
April 2002, represents the first true diffraction-limited coronagraph,
and is optimized to take advantage of the high-order adaptive
optics of the AEOS telescope. The
instrument incorporates a number of novel coronagraphic features,
including high speed image alignment and stability loops: both
components which are critical to achieving the level of contrast
necessary for imaging exoplanets and dust disks.

The images below show the opto-mechanical design of the Lyot Project
coronagraph, both from a top-down view and a three-dimensional
perpective. The entire corongraph assembly fits on a 4 x 4 foot
"breadboard", fitted to an optical bench that keeps the surface
horizontal, and enlosed in a sealed aluminium enclosure to
prevent air currents from affecting the propagation of light through
the instrument.

Figure
1: The Lyot Project coronagraph under construction at the American
Museum of Natural History in New York.

Light Propagation

The 4.1-inch beam from the AEOS telescope enters the instrument from
the right of Figure 2. The beam is indicated
by the dark blue rays that trace the light path through this drawing.
The beam first impinges upon the beam capture mirror (BCM), a 6-inch
flat mirror that directs the beam
to the subsequent optics. The off-axis parabolae (OAP1
and OAP2) are two
custom, gold-surfaced mirrors (built by Axsys Technologies) which serve to compress the large AEOS
beam to more manageable 10.48mm diameter. The Fast Steering Mirror
(FSM) is the first stage of the instrument's
tip-tilt loop, allowing the star to be placed with great accuracy on
the coronagraphic occulting disk (see the section on Coronagraphy), which is
marked FPM (for focal plane mask) in the drawing. The optical design
may seem complex, but the principal purpose in using so many
reflections is to create the image and pupil planes that are required
for a coronagraph. The Lyot stop is marked Lyot in Figure
2. After
the Lyot stop, the beam is reflected upward onto a final focusing
element called OAP3, which reflects the light into the Kermit Camera
(Dewar). Kermit, itself was constructed with a similar PDR/CDR
process. More about Kermit is available at the Kermit web page.